1. Field of the Invention
The present invention relates to implantable medical devices. More specifically, the present invention relates to implantable medical devices that deliver therapy to intervene in sleep-disordered breathing cycles.
2. Description of the Related Art
Sleep-disordered breathing encompasses a variety of conditions including, for example, central sleep apnea and obstructive sleep apnea. In both instances, a cessation of breathing (apnea) and/or shallow or slow breathing (hypopnea), occurs on an intermittent or periodic basis. The apnea is generally terminated by an arousal from sleep leading to a disturbance of the sleep pattern. Such interrupted sleep often causes excessive daytime sleepiness, depression, irritability, memory impairment, and headaches. In addition, sleep-disordered breathing can be life threatening and an increasing correlation between sleep-disordered breathing and hypertension, diabetes, stroke, arrhythmia, heart failure and heart attacks is being established.
Obstructive sleep apnea is the result of a blockage of a portion of the upper airway, usually associated with a relaxation in muscle tone and/or a reduction in the size of the airway due to, for example, excessive fatty tissue. This mechanical blockage creates a pressure differential that further facilitates the apnea.
Central sleep apnea is a neurological disorder, wherein normal breathing patterns are interrupted due to a failure of the brain to generate the proper muscle stimulation pulses. Once initiated, the resultant apnea is terminated with a resumption of respiration. Central sleep apnea can precede obstructive sleep apnea and this combination is referred to as mixed sleep apnea.
One particular variant of central sleep apnea that is often associated with patients suffering from chronic heart failure is Cheyne-Stokes respiration. Cheyne-Stokes respiration is a pattern of breathing characterized by a waxing and waning of tidal volume with complete cessation of breathing. Typically, a cycle of Cheyne-Stokes respiration lasts about 30–90 seconds. The cycle then repeats itself.
There are a variety of treatment options available for addressing sleep-disordered breathing. The most common treatment is the use of CPAP (Continuous Positive Airway Pressure). The patient generally wears an appliance such as a full facemask or more typically a nose covering mask or nasal inserts that deliver pressurized air into the airway to maintain the airway in an open state. The therapy is effectively a pneumatic stent. CPAP is generally effective at treating both central and obstructive apnea; however, many patients do not tolerate the therapy and discontinue its use.
Another method used to address sleep-disordered breathing involves atrial overdrive pacing (AOP). That is, an implantable medical device (IMD) is implanted to pace the heart. The pacing rate is elevated from a normal resting or sleeping rate (e.g., normal nocturnal intrinsic rate or a normal paced sleeping rate). For example, such techniques are described in U.S. Pat. No. 6,126,611, assigned to Medtronic, Inc., which is herein incorporated by reference in its entirety.
There are several timing scenarios for initiating AOP. For example, AOP can be delivered anytime a sleep state is detected or at predetermined intervals during sleep. Alternatively, AOP can be delivered when apnea is detected.
Similarly, there are various mechanisms for delivering specific AOP therapies. For example, once initiated the overdrive pacing can be set to a given higher rate, such as for example 3–5 beats/minute above normal, 10–12 beats/minute above normal, or even higher rates such as 30 beats/minute above normal. Alternatively, AOP can be initiated at a lower level above normal and gradually increased.
The use of AOP to address sleep-disordered breathing and its exact mechanisms are currently being investigated. The elevated pacing rate may lead to arousal such that apnea is terminated; however, it is believed more likely that the maintenance of heart rate and the increase in cardiac output achieved via AOP may positively affect autonomic tone sufficiently to reduce the overall number of apneas. While promising, the use of AOP does not always provide a successful therapy for sleep-disordered breathing.